The present invention relates generally to a method and an apparatus for controlling an electromagnetic clutch for use on a vehicle. More particularly, the invention is concerned with improved technologies for controlling an engaging operation of such an electromagnetic clutch upon starting or during running of the vehicle, without the engaging operation being affected by a variation in torque transmission characteristics between the clutches used on different vehicles, and permitting the individual clutches to provide a consistent power transmission performance, while assuring an equally high level of drivability and fuel economy on the different vehicles.
In the art of a power transmission system of a vehicle, an electromagnetic clutch is used as means for easy control of a torque to be transmitted. An example of such an electromagnetic clutch comprises: a driving rotary member operatively connected to an engine of a vehicle; a driven rotary member operatively connected to drive wheels of the vehicle and cooperating with the driving rotary member to define an annular space therebetween; a mass of magnetic powder accommodated in the annular space between the driving and driven rotary members; and a solenoid which is energized to generate a magnetic force for causing particles of the magnetic powder to be tightly bonded together in the annular space so that the mass of magnetic powder contacts the driving and driven rotary members to transmit a torque from the driving rotary member to the driven rotary member. In this electromagnetic clutch, a value of the torque which is delivered from the driving rotary member (from the engine) to the driven rotary member (drive wheels) is varied as a function of an electric current to be applied to the solenoid. This characteristics of the electromagnetic clutch is utilized to obtain an optimum control of an engaging action of the clutch, at the time of starting the vehicle for example, by increasing a magnitude of electric current to be applied to the solenoid as the engine speed is increased.
The above method of controlling the engaging operation of the electromagnetic clutch is generally advantageous in drivability and specific fuel consumption (fuel economy) of the vehicle. Commonly, the increase in the energization current of the solenoid is controlled relative to the increase in the engine speed according to a predetermined relation between these two variables. For instance, the energization current to be supplied to the solenoid is varied in proportion to a difference between a preset idling speed of the engine and an actual running speed of the engine. In this method, a variation in torque transmission characteristics between clutches on different vehicles will cause an inconsistency in engaging operation between the clutches. Stated in more detail, the torque transmission characteristics in relation to the energization current of the solenoid are inevitably varied from one clutch to another, due to difference in dimensions of the annular space formed between the driving and driven rotary members (the difference being caused due to dimensional tolerances), and due to differences in operating temperature and cumulative operation time of the clutches. Since the variation in the transmission characteristics for such causes is inevitable, it is a common practice that a rate of increase in the solenoid energization current is set at a level which is low enough to assure a high or sufficient drivability of each vehicle, even when the transmission characteristics of the electromagnetic clutch of that vehicle is inclined on the side of lowering the drivability of the vehicle. As a result of the above setting of the increase rate of the solenoid current, the running speed of the engine during an engaging action (partial engagement) of the electromagnetic clutch tends to be unnecessarily higher on the side of causing a loss of output of the engine. Thus, the traditional method and apparatus for controlling the engaging action of an electromagnetic clutch are not considered sufficiently satisfactory in terms of specific fuel consumption, i.e., fuel economy, particularly at the time of starting the vehicle.